In current control for an electromagnet of a magnetic bearing, it is difficult to drastically to change energizing current for floating a rotor, since the inductance of the exciting coil of the bearing is high.
In order to ensure good responsiveness of a chopper circuit for the energizing current of an inductance load, the supply voltage is increased correspondingly if the inductance load is great, so that the rise of the energizing current is quickened, and accumulated magnetic energy attributable to the inductance is returned to the power supply. By doing this, the fall of the energizing current is quickened to improve responsiveness, whereby the energizing current is controlled.
There are means for increasing the supply voltage which are used to ensure high-speed response of the energizing current of a great inductance load by means of a chopper circuit. For higher responsiveness, however, the supply voltage is inevitably increased to an unpractically high level.
In the case of an induction motor of 500-W output or a reluctance-type electric motor, for example, a voltage five to ten times as high as the necessary applied voltage for the driving torque is needed to obtain a pulse width of several microseconds or thereabout for the chopper control of the armature current supply, which is not practical.